Butt Welding Using Medium Power Fiber Lasers

In many end-user applications, a butt-welding configuration is required (for functional / aesthetic reasons). Although in process terms it requires the lowest heat input, permits the smallest HAZ, and the fastest speed, butt welding places stringent requirements in terms of the dimensional accuracy, tolerances and part-to-part match of the surfaces to be welded, the tooling, fixturing accuracy, repeatability of the welding equipment, and the in-process control of beam guiding system.

Medium-power optical Fiber Lasers are well-suited to precision welding of metals across a diverse range of applications.  In this application insight we discuss generic welding applications and illustrate these with two case studies in butt-welding.

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Butt welding using fiber lasers

As a general guideline for butt welding, gaps in the joint between mating parts must be less that ~50% of the focused beam diameter to avoid the beam passing through the “aperture” in the butt joint without welding it at all. As noted above, for reliable welding, the weld tooling must hold the parts in close contact and present the seam consistently to the Laser beam; optical imaging systems are increasingly used to ensure that the Laser beam aligns to and tracks the required weld path.

In situations where this level of precision cannot be assured because the seam location is not controlled accurately enough, a lap joint can often be utilised to provide a much larger tolerance on alignment.  The only requirement on a lap weld is that piece parts are in contact and that the beam alignment and tracking impinges on this zone.

Butt welding of Stainless Steel Sheet

Figure 4: Process fixture for butt welding trials. Pre-welded samples with 50µm gap

Figure 4: Process fixture for butt welding trials. Pre-welded samples with 50µm gap

304 grade stainless steel sheet used in the manufacture of domestic goods was welded using a 75µm spot size. Trials were undertaken with controlled butt gaps and with both shear-cut and Laser-cut materials.  Figure 4a and 4b below shows the test arrangement and a through-the-head image of pre-welded samples respectively.

Figure 4: c) Top surface of weld bead; d) Bottom surface of weld bead Samples were welded with zero nominal gap

Figure 4: c) Top surface of weld bead; d) Bottom surface of weld bead Samples were welded with zero nominal gap

Sample welds are shown above for top and bottom welds in 0.5mm sheet, made with the piece parts butted.

The process speed was in excess of 2 m/min, and was deliberately reduced from the “bead-on-plate” maximum to increase the width of the fusion zone to ensure full fusion throughout the full depth of the weld.

Butt welding of Chromium Steel Coil Strip

Plated steel coil strip is extensively used in the manufacture of a diverse range of products:  Stainless steel strip is used in the manufacture of seam-welded tubes. Chromium-plated steel strip is used in elongated cylindrical items such as protective sheaths on a wide range of cables, the steel sheath providing both increased mechanical strength and also rodent protection. In the manufacture of such products, processes can run continuously for several hours and may require in-process, on-the-run, tape-to-tape welding of coil strip batches.  Typically the time available to introduce a new coiled roll and to weld it to the tail-end of the previous roll is less than two minutes; an excess-length tape-accumulator is used to provide “buffer” between coils and to enable the weld joint to be made in stationary manner.

As the weld joining the tape sections will itself be processed and incorporated into the finished item, the in-line tape joints must have high dimensional uniformity and mechanical integrity, and must generally be butt-welded. Tape materials with thicknesses in the range 50µm to 250µm are utilised according to application and specification requirements; thicker tapes provide greater protection but impose limitations in terms of flexibility and bend radius.

Weld joint quality must satisfy the following criteria:

  • Uniform flat seam
  • No porosity
  • Good edge quality (no “notches” at weld edge)

Crucial to the success of the welding process is the set-up tooling and the edge to edge match of the two tapes, both laterally and vertically (“bow”).

Whilst the tape ends can be cut and prepared for welding using a precision shear, an optional alternative solution (free of the need for regular maintenance / shear-blade change-out) is to use the same Laser to cut the tapes on the tool prior to welding them together. The Laser-cut edge provides improved dimensional accuracy, free of distortion, and repeatable quality run-to-run, week-to-week.

Figure 5 a & b: Close up of weld in 200um tape showing weld dimensions

Figure 5 a & b: Close up of weld in 200um tape showing weld dimensions

Figure 5 (above) shows images of an example weld in 15mm wide, 0.2mm thick tape joint. The weld was made at only 200W power at a speed of 1.2m/min; a low flow of argon gas was used to provide a controlled environment for the weld.  Note that the incident beam was defocused (+3mm) in order to increase the spot-size at the work piece. The seam weld is made using two “dummy tapes” alongside the work pieces so that the seam weld can be over-run into the dummy tapes to avoid “notching” at the edge of the tape;  the dummy tapes are readily detached from the work piece tape after welding.

Benefits of the SPI Fiber Laser:

  • Energy efficiency
  • Maintenance free
  • Compact design
  • Low capital and operational costs.

Click here to view our range of datasheets including the 500W single mode, Multimode 50µm and Multimode 100µm Lasers.

Please contact our Lead Qualification Engineer for advice and further details on machine builders who have the capabilities of creating a Fiber Laser based welding station.  Greg.thomas@spilasers.com

Related Product – redPOWER

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